Heart failure is a condition wherein the heart fails to circulate blood sufficiently to meet the metabolic demands of the various tissues of the body. In other words, cardiac output is insufficient to satisfy the body.
Cardiac output is influenced by the stroke volume exhibited by the heart. For a given heart, stroke volume may be considered a function of preload and inotropic state. Briefly, preload refers to the stretching of the myocardial cells in a chamber (e.g., left ventricle) during diastole, before contraction of the chamber. Preload may be measured as the end-diastolic volume or pressure exhibited by the blood within the chamber in question. Inotropic state refers generally to the hormonal milieu exerting influence upon the heart. The inotropic state of a heart determines the strength of its next contraction.
A Frank Starling curve presents the relationship between stroke volume, preload, and inotropic state for a given heart. An exemplary Frank Starling curve is depicted in FIG. 1. The chart of FIG. 1 is plotted on a Cartesian plane, with stroke volume measured along the y-axis, and preload measured along the x-axis. Three solid curves 100, 102, and 104 are depicted on the chart. Each curve 100, 102, and 104 corresponds to a different inotropic state. Curve 100 corresponds to inotropic state S1, curve 102 corresponds to inotropic state S2, and curve 104 corresponds to inotropic state S3.
The inotropic state exhibited by a person's heart may vary with exertion or emotional state, among other factors. Thus, for example, inotropic state S1 (curve 100) may represent the inotropic state of a particular person's heart during ordinary waking non-strenuous activity. State S2 (curve 102) may represent the inotropic state of the person's heart during strenuous exertion, and state S3 (curve 104) may represent the inotropic state of the person's heart during rest. As can be seen from FIG. 1, when the inotropic state exhibited by a heart elevates, a greater stroke volume is yielded for a given level of preload (because the hormonal influences upon the heart cause the heart to contract more forcefully). Conversely, when the inotropic state exhibited by a heart depresses, a lesser stroke volume is yielded for a given level of preload.